This is why you ask questions to the employees in the stores. If it doesn't sound plausible then you may not want to believe anything else they tell you.
Also most of these pet stores do add some sort of medication to their water but depends on the person in charge of the department what type of routine they will follow.
If you plan to, "Quarantine
" 4 days will do basically NOTHING.
- 2 weeks is recommended, but 4 weeks is the industry standard
The people I know that follow the "standard" are some dedicated fishkeepers who are into the saltwater varieties who order very EXPENSIVE (anything over $50 & up) species & will QT a new "specimen" in a fully established & fully equipped hospital tank for a little over a month.